The human APOBECSG and APOBECSF (ASG/F) proteins restrict the ability of retroviruses, including HIV-1, to successfully infect cells. This defense is caused by deamination of nascent dC residues, converting them to dU in the cDNA strand of the replicating HlV-1. The resulting mutated viral DNA leads to functional inactivation of the invading virus. As a counter-defense, HIV-1 and other lentiviruses express Vif which mediates the degradation of ASs by the cellular proteasome machinery. HlV-1 replication is completely blocked when the interaction between ASG/Vif is prevented, pointing at the potentiality of ASG as an antiviral agent. We therefore speculate that preventing ASG/F/Vif interaction or enhancing A3G/F activity in the cells may prevail over Vif activity, enabling the host cell defense machinery to halt the production of infectious virus. The overall goal of this study is to define small molecules, which will allow ASG/F to neutralize HlV-1 in its natural target cells. A better understanding of the mechanism by which A3G/F hypermutates the viral DNA and the way(s) Vif counteracts this activity is essential for designing these compounds. Our specific aims are to (i) determine the impact of ASG/F oligomerization on deaminase activity and other biochemical properties, (ii) determine the biochemical properties of ASG/F interaction with single-stranded DNA, and (iii) evaluate compounds that modulate ASG/F/Vif interactions. These aims will be achieved by applying biochemical and biophysical approaches;and particulariy a novel quantitative DNA deaminase assay, to understand the DNA deaminase activity of ASG/F and ASG/F variants, and by analyzing the activity of A3G/F in the presence/absence of Vif and/or Vif fragments/peptides.